In the field of photographic processing it is well known to replenish processing solutions to compensate for loss of developer components by consumption by the process and aerial oxidation. Such a replenishment system requires replenisher pump(s), pipework and control means, all of which adds to the cost of the machine.
Some processing machines can be supplied by premixed solutions that are usually run until they produce unacceptable results. At this point the solutions are discarded and are never replenished. This clearly avoids the replenishment system described above.
The problem experienced with such non-replenished machines is that the quality of the images produced will deteriorate with continued use of the same solution. This means that the processing solutions must be discarded at a comparatively early stage if unvarying high quality processing is desired.